I bought a gently used upright Lifecycle 12 years ago for cross-training twice a week, and it still works great. Personally I prefer an upright bike to a recumbent - I actually find recumbents less comfortable, and the upright feels much more like a "real" bike. Definitely test before you buy. However, in general, bikes tend to work primarily the muscles in front of your legs (especially quads) whereas running works the muscles in back of your legs (especially hamstrings). In other words, a bike will build your aerobic capacity but not your most important running muscles. If you're looking for a closer approximation to running, but without the impact, I'd recommend an elliptical trainer. You can then do leg lifts with ankle weights a few times a week to very effectively build up your quads.